Currently available high-performance fiber-optic gyros (FOG) typically employ lithium niobate modulators. Lithium niobate (LN) is the material of choice for high-bandwidth modulation of the optical phase waveform to allow closed-loop operation of the gyroscope and to enable scale factor nonlinearity on the order of parts per million (ppm) over wide dynamic ranges. It is known that lithium niobate modulators suffer from crystal defects, which produce sensitivities to environment, especially vacuum or other gas exposures as is shown in U.S. Pat. No. 7,228,046 to Hendry et al. entitled ENVIRONMENTALLY STABLE ELECTRO-OPTIC DEVICE AND METHOD FOR MAKING SAME. For this reason, some FOG designs employ a hermetic seal with oxygen-based backfill gases to stabilize LN defects that might otherwise be destabilized by vacuum or other gas exposure.
In standard terrestrial uses of LN phase modulators, the devices are typically immersed in 1-atmosphere air. As known to one skilled in the art, air includes about 20.9% oxygen. It is also known that the conductivity of lithium niobate increases with decreasing oxygen partial pressure (see W. Jorgensen and R. W. Bartlett, J. Phys. Chem Solids 30 (12), 2639, 1969). In other words, exposure to the oxygen in air keeps the conductivity of LN low. Defects in the waveguides of LN modulators, especially waveguides manufactured by the process of annealed proton exchange (APE) can contribute to environmentally-induced changes in conductivity as is shown in U.S. Pat. No. 8,189,981 to Muller et al. entitled STABLE LITHIUM NIOBATE WAVEGUIDES, AND METHODS OF MAKING AND USING SAME. Exposure to oxygen serves to stabilize these defects and prevent changes in annealed proton exchange LN performance.
When a LN phase modulator has a low band gap, the modulator has electrical leakage. In a vacuum, the uncoordinated sites in the lithium niobate material are not filled, which causes the band gap to drop to a low level that is below the band gap of lithium niobate material in O2.
Another problem for currently available lithium niobate phase modulators may occur when other components near the lithium niobate phase modulator outgas. Some outgassed species may degrade the LN bandgap even further.